Gamma ray camera for nuclear medicine

ABSTRACT

An improved gamma camera arrangement wherein the spatially defined acceptance of gamma radiation from a source distribution of clinical interest is enhanced by a unique collimator structure. Aliasing frequencies otherwise derived from an orthogonal strip detector array are identified and avoided through a structural geometry wherein collimator resolution is greater than or equal to about 1.7(l), where, l, is the strip spacing of the array.

The invention described herein was made in the course of work under agrant or award from the Department of Health, Education and Welfare.

BACKGROUND

The field of nuclear medicine has long been concerned with techniques ofdiagnosis wherein radio pharmaceuticals are introduced into a patientand the resultant distribution or concentration thereof as evidenced bygamma ray intensities is observed or tracked by an appropriate system ofdetection. An important advantage of the diagnostic procedure is that itpermits non-invasive investigation of a variety of conditions of medicalinterest. Approaches to this investigative technique have evolved fromearly pioneer procedures wherein a hand-held radiation counter wasutilized to map body contained areas of radioactivity to more currentsystems for imaging gamma ray source distributions, in vivo. Ininitially introduced practical systems, scanning methods were providedfor generating images, such techniques generally utilizing ascintillation-type gamma ray detector equipped with a focusingcollimator which moved continuously in selected coordinate directions,as in a series of parallel sweeps, to scan regions of interest. Adrawback to the scanning technique resides in the necessarily longerexposure times required for the derivation of an image. For instance,such time elements involved in image development generally are overlylengthy to carry out dynamic studies of organ function.

By comparison to the rectilinear scanner described above, the laterdeveloped "gamma camera" is a stationary arrangement wherein an entireregion of interest is imaged at once. As initially introduced thestationary camera systems generally utilized a larger diameter sodiumIodide, Na I (Tl) crystal as a detector in combination with a matrix ofphotomultiplier tubes. For additional information concerning suchcamera, see:

I. anger, H. O., "A New Instrument For Mapping Gamma Ray Emitters,"Biology and Medicine Quarterly Report UCRL-3653, 1957.

A multiple channel collimator is interposed intermediate the sourcecontaining subject of investigation and the scintillation detectorcrystal. When a gamma ray emanating from the region of investigativeinterest interacts with the crystal, a scintillation is produced at thepoint of gamma ray absorption and appropriate ones of thephotomultiplier tubes of the matrix respond to the thus generated lightto develop output signals. The original position of gamma ray emanationis determined by position responsive networks associated with theoutputs of the matrix.

A continually sought goal in the performance of gamma cameras is that ofachieving a high resolution quality in any resultant image.Particularly, it is desirable to achieve this resolution in combinationwith concomitant utilization of a highly versatile radionuclide orradiolabel, 99m-Technetium, having a gamma ray or photon energy in theregion of 140 keV.

The resolution capabilities of gamma cameras incorporating scintillationdetector crystals, inter alia, is limited both by the light couplingintermediate the detector and phototube matrix or array as well as byscatter phenomena of the gamma radiation witnessed emanating from withinthe in vivo region of investigation. Concerning the latter scatteringphenomena, a degradation of resolution occurs from scattered photonswhich are recorded in the image of interest. Such photons may derivefrom Compton scattering into trajectories wherein they are caused topass through the camera collimator and interact photoelectrically withthe crystal detector at positions other than their point of in vivoderivation. Should such photon energy loss to the Compton interaction beless than the energy resolution of the system, it will effect anoff-axis recordation in the image of the system as a photopeak photonrepresenting false spatial information or noise. As such scatteredphotons record photopeak events, the noise increase and consequentresolution quality of the camera diminishes. For the noted desirable 140keV photons, the scintillation detector-type camera energy resolution isapproximately 22 keV. With this resolution, photons which scatterthrough an angle from 0° to about 70° will be seen by the system as suchphotopeak events.

A continuing interest in improving the resolution qualities of gammacameras has lead to somewhat extensive investigation into imagingsystems incorporating relatively large area semiconductor detectors.Such interest has been generated principally in view of theoreticalindications of an order of magnitude improvement in statisticallylimited resolution to provide significant improvements in image quality.In this regard, for example, reference may be made to the followingpublications:

Ii. r. n. beck, L. T. Zimmer, D. B. Charleston, P. B. Hoffer, and N.Lembares, "The Theoretical Advantages of Eliminating Scatter in ImagingSystems," Semiconductor Detectors in Nuclear Medicine, (P. B. Hoffer, R.N. Beck, and A. Gottschalk, editors), Society of Nuclear Medicine, NewYork, 1971, pp. 92-113.

Iii. r. n. beck, M. W. Schuh, T. D. Cohen, and N. Lembares, "Effects ofScattered Radiation on Scintillation Detector Response, "MedicalRadioisotope Scintigraphy, IAEA, Vienna, 1969, Vol. 1, pp. 595-616.

Iv. a. b. brill, J. A. Patton, and R. J. Baglan, "An ExperimentalComparison of Scintillation and Semiconductor Detectors for IsotopeImaging and Counting," IEEE Trans. Nuc. Sci., Vol. NS-19, No. 3, pp.179-190, 1972.

V. m. m. dresser, G. F. Knoll, "Results of Scattering in RadioisotopeImaging," IEEE Trans. Nuc. Sci., Vol. NS-20, No. 1, pp. 266-270, 1973.

Particular interest on the part of investigators has been paid todetectors formed as hybridized diode structures fashioned basically ofgermanium. To provide discrete regions for spatial resolution ifimpinging radiation, the opposed parallel surfaces of the detectordiodes may be grooved or similarly configured to define transverselydisposed rows and columns, thereby providing identifiable discreteregions of radiation response. Concerning such approaches to treatingthe detectors, mention may be made of the following publications:

Vi. j. detko, "Semiconductor Dioxide Matrix for Isotope Localization,"Phys. Med. Biol., Vol. 14, No. 2, pp. 245-253, 1969.

Vii. j. f. detko, "A Prototype, Ultra Pure Germanium Orthogonal StripGamma Camera," Proceedings of the IAEA Symposium on RadioisotopeScintigraphy, IAEA/SM-164/135, Monte Carlo, October 1972.

Viii. r. p. parker, E. M. Gunnerson, J. L. Wankling, and R. Ellis, "ASemiconductor Gamma Camera with Quantitative Output," MedicalRadioisotope Scintigraphy.

Ix. v. r. mcCready, R. P. Parker, E. M. Gunnerson, R. Ellis, E. Moss, W.G. Gore, and J. Bell, "Clinical Tests on a Prototype SemiconductorGamma-Camera," British Journal of Radiology, Vol. 44, 58-62, 1971.

X. parker, R. P., E. M. Gunnerson, J. S. Wankling, R. Ellis, "ASemiconductor Gamma Camera with Quantitative Output," MedicalRadioisotope Scintigraphy, Vol. 1, Vienna, IAEA, 1969, p. 71.

Xi. detko, J. F., "A Prototype, Ultra-Pure Germanium, orthogonal-StripGamma Camera," Medical Radioisotope Scintigraphy, Vol. 1, Vienna, IAEA,1973, p. 241.

Xii. schlosser, P. A., D. W. Miller, M. S. Gerber, R. F. Redmond, J. W.Harpster, W. J. Collis, W. W. Hunter, Jr., "A Practical Gamma Ray CameraSystem Using High Purity Germanium," presented at the 1973 IEEE NuclearScience Symposium, San Francisco, November 1973; also published in IEEETrans. Nucl. Sci., Vol. NS-21, No. 1 February 1974, p. 658.

Xiii. owen, R. B., M. L. Awcock, "One and Two Dimensional PositionSensing Semiconductor Detectors," IEEE Trans. Nucl. Sci., Vol. NS-15,June 1968, p. 290.

In the more recent past, investigators have shown particular interest informing orthogonal strip matrix detectors from p-i-n semiconductorsfashioned from an ultra pure germanium material. In this regard,reference is made to U.S. Pat. No. 3,761,711 as well as to the followingpublications:

Xiv. j. f. detko, "A Prototype, Ultra Pure Germanium, Orthogonal StripGamma Camera," Proceedings of the IAEA Symposium on RadioisotopeScintigraphy, IAEA/SM-164/135, Monte Carlo, October, 1972.

Xv. schlosser, P. A., D. W. Miller, M. S. Gerber, R. F. Redmond, J. W.Harpster, W. J. Collis, W. W. Hunter, Jr., "A Practical Gamma Ray CameraSystem Using High Purity Germanium," presented at the 1973 IEEE NuclearScience Symposium, San Francisco, November 1973; also published in IEEETrans. Nucl. Sci., Vol. NS-21, No. 1, February 1974, p. 658.

High purity germanium detectors promise numerous advantages both ingamma camera resolution as well as in economic feasibility orpracticality. For instance, by utilizing high purity germanium as adetector, lithium drifting arrangements and the like for reducingimpurity concentrations are avoided and the detector need only be cooledto requisite low temperatures during its clinical operation. Readoutfrom the orthogonal strip germanium detectors is described as beingcarried out utilizing a number of techniques, for instance, each stripof the detector may be connected to a preamplifier-amplifier channel andthence directed to an appropriate logic function and visual readout. Inanother arrangement, a delay line readout system is suggested with theintent of reducing the number of preamplifiers-amplifier channels, and atechnique of particular interest utilizes a charge splitting method.With this method or technique, position sensitivity is obtained byconnecting each contact strip of the detector to a charge dividingresistor network. Each end of each network is connected to a virtualearth, charge sensitive preamplifier. When a gamma ray interacts withthe detector, the charge released enters the string of resistors anddivides in proportion to the amount of resistance between its entrypoint in the string and the preamplifiers. Utilizing fewerpreamplifiers, the cost and complexity of such systems is advantageouslyreduced. A more detailed description of this readout arrangement isprovided in:

Xvi. gerber, M. S., Miller, D. W., Gillespie, B., and Chemistruck, R.S., "Instrumentation For a High Purity Germanium Position Sensing GammaRay Detector," IEEE Trans. on Nucl. Sci., Vol. NS-22, No. 1, February,1975, p. 416.

To achieve requisite performance and camera image resolution, it isnecessary that substantially all sources of noise or false informationwithin the system be accounted for. In the absence of adequate noiseresolution, the performance of the imaging systems may be compromised tothe point of impracticality. Until the more recent past,charge-splitting germanium detector arrangements have not beenconsidered to be useful in gamma camera applications in consequence ofthermal noise anticipated in the above-noted resistor divider networks,see publication VII, supra. However, as will be evidenced in thedescription to follow, such considerations now are moot.

Another aspect in the optimization of resolution of the images of gammacameras resides in the necessarily inverse relationship betweenresolution and sensitivity. A variety of investigations have beenconducted concerning this aspect of camera design, it being opined thatphoton noise limitations, i.e. statistical fluctuations in the image,set a lower limit to spatial resolution. Further, it has been pointedout that the decrease in sensitivity witnessed in conventional highresolution collimators may cancel out any improvements sought to begained in image resolution. A more detailed discourse concerning theseaspects of design are provided, for instance, in the followingpublications:

Xvii. e. l. keller and J. W. Coltman, "Modulation Transfer andScintillation Limitations in Gamma Ray Imaging," J. Nucl. Med. 9, 10,537-545 (1968).

Xviii. b. westerman, R. R. Sharma, and J. F. Fowler, "RelativeImportance of Resolution and Sensitivity in Tumor Detection," J. Nucl.Med. 9, 12, 638-640 (1968).

More recent investigation of gamma camera performance has identifiedstill another operational phenomenon tending to derogate from spatialresolution quality. This phenomenon is referred to as "aliasing" andrepresents a natural outgrowth of the geometry of the earlier notedorthogonal strip germanium detector. The phenomenon further represents asubject to which the instant invention will be seen, inter alia, to beconsidered in detail. A more detailed discussion of the phenomena isprovided at:

Xix. j. w. steidley, et al., "The Spatial Frequency Response ofOrthogonal Strip Detectors," IEEE Trans. Nuc. Sci., February, 1976.

SUMMARY

The present invention is addressed to an improved gamma cameraarrangement wherein the spatially defined acceptance of radiationemanating from a regionally disposed source is enhanced to achievesignificant improvement in imaging resolution. The invention recognizesa highly important operational aspect of gamma cameras utilizingorthogonal strip germanium detectors as residing in the geometricallyinherent creation of spurious spatial frequencies. These frequencies,termed "aliasing frequencies" have been determined to result from thesampling performance of the noted orthogonal strip type detector andtheir influence upon such camera systems ultimately may be found todictate the resolution capacities of imaging systems utilizing the noteddetector arrangement.

An important aspect and object of the invention is to provide, in agamma camera incorporating an orthogonal strip array semiconductordetector for deriving spatial and energy level information correspondingwith a source distribution, an improved collimator arrangement servingto limit the frequency content of the source reaching the detector tofrequencies less than about one half of the sampling frequency in onedimension of the strip array or matrix of the detector.

As another aspect and object of the invention, an improved collimatorarrangement is provided for a gamma camera of the type described havingan orthogonal strip array semiconductor detector wherein the collimatorarrangement is present as an array of adjacently disposed channelshaving sides defining a square cross-section and aligned normally to thereceiving surface of the detector. The array of channels is configuredto define a septal thickness, T, intermediate the channels. Further, theeffective thickness of the collimator, represented as A_(E), exhibitedwherein A_(E) = A - 2/μ (E), where μ (E) is the attenuation coefficientof the surface material of the channels of the collimator for a givenenergy level E of a radiation source. The collimator is formed toprovide a collimator resolution, R_(c), equal to or greater than about1.7 (l) and is configured in substantial satisfaction of the expression:

    R.sub.c = (D/A.sub.E) (A + B + C)

where C is the distance from the inwardly disposed plane defining sideof the collimator to the midplane of the orthogonal strip detector; A isthe thickness of the collimator and B is the distance between the sourceof radiation and the outwardly disposed plane defining side of thecollimator; D is the effective diameter of the noted channels; and l isthe strip spacing of the orthogonal strip array detector.

The invention further contemplates as an object, the improved gammacamera arrangement described above wherein the septal wall thickness, T,of the noted collimator is equal to or about: ##EQU1## wherein, P, isthe penetration fraction of the side defining material of thecollimator, this fraction having a value about equal to or less than0.05.

As another feature in object of the invention, a collimator arrangementfor a gamma camera system is provided, which in addition to exhibitingthe advantageous operational characteristics described above, isfabricable utilizing sheet tungsten or tantalum or side channel materialin consequence of a geometric configuration rendering its manufacturepractical. In this regard, the array of channels making up thecollimator arrangement comprises a plurality of sheet members each beingconfigured having a plurality of mutually equally spaced parallel slotsof length equal to or about the height of the sheets and the concomitantcollimator thickness. The slots are formed having a width correspondingwith the thickness of the sheets themselves. The sheet members, once soconfigured, are assembled by being mutually internested to define thecollimator array. Preferably, the slots are formed within the sheetmember by chemical milling techniques to achieve requisite tolerancesfor effecting control over streaming phenomena otherwise encountered inthe performance of the camera system. Through the use of tungsten ortantalum components the septum thickness of the collimator array may beadvantageously reduced for improved system efficiency, particularly fordesired radiation energy levels, for instance, in the range of about 140keV. The noted streaming phenomena may be contained within acceptablelevels by maintaining the noted slot tolerances equal to or less thanabout 0.001 inch.

The invention further contemplates, as an object, the gamma cameraarrangement and system as set forth hereinabove wherein, for acollimator resolution as above defined, the multichannel collimatorarray is configured having an optimized collimator geometric efficiency,φ_(s), where: ##EQU2## for a channel cross-section of squareconfiguration.

As another object, the invention contemplates a collimator arrangementfor a gamma camera assembly of a variety operative to derive imagedefining information of the source distribution of gamma rays, suchcamera incorporating an orthogonal strip array semiconductor detector,the strips of which have a spatial frequency, l/c, and center-to-centerstrip spacing, l. This collimator perferably is formed of a plurality ofsheet members of height, h, each of the members being configured havinga select number of parallel slots extending from one edge thereof alongthe dimension of the height. The slots extend a distance substantiallyequivalent to one half the height, h, and are mutually spaced in aregularly recurring manner to define a slot-to-slot pitch substantiallyequivalent to the side cross sectional dimension of one chamber withinthe collimator plus a predetermined tolerance dimension. Any two of thenoted sheet members are joined by being mutually internested aboutselect ones of the slots of each to form the parallel multiple channelcollimator assembly.

Other objects of the invention will, in part, be obvious and will, inpart, appear hereinafter.

The invention, accordingly, comprises the system and apparatuspossessing the construction, combination of elements and arrangement ofparts which are exemplified in the following detailed disclosure.

For a fuller understanding of the nature and objects of the invention,reference should be had to the following detailed description taken inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a gamma camera arrangement asmay utilize the improvements of the invention, showing, in blockschematic form, control functions of the system;

FIG. 2 is a pictorial representation of a solid state orthogonal striphigh purity germanium detector incorporating a charge splitting resistornetwork in combination with preamplification electronics;

FIG. 3 is a schematic representation of a solid state orthogonal stripdetector and a schematic collimator functionally associated therewith assuch system components relate to a radiation source within a region ofclinical interest;

FIGS. 4a - 4c are a schematic and graphical representation of thefundamental geometry associated with the interrelationship of amulti-channel collimator and a solid state detector as utilized inaccordance with the present invention;

FIG. 5 is a pictorial representation of a collimator array fabricated inaccordance with a preferred embodiment of the instant invention;

FIG. 6 is a pictorial view of two internested members of the collimatorof FIG. 5;

FIGS. 7a - c respectively and schematically depict representations of asource distribution as related with the geometry of an orthogonal stripdetector and image readouts for illustrating aliasing phenomena;

FIGS. 8a - d portray vertically aligned graphs relating modulationtransfer function with respect to resolution as such data relates toaliasing phenomena,

FIG. 8a showing collimator modulation transfer function (MTF_(c)) withFWHM resolution of 1.33 l,

FIG. 8b showing a consequent alias frequency spectrum which is processedby the electronics of the camera system,

FIG. 8c showing electronic MTF for given resolutions, and

FIG. 8d showing camera system MTF's revealing aliasing introduced by theorthogonal strip solid state detector; and

FIGS. 9a - d provide curves showing the results of aliasing correctionas compared with the curves of FIGS. 8a - d,

FIG. 9a looking to collimator design as an anti-aliasing filter;

FIG. 9b showing a consequent aliasing frequency spectrum which isprocessed by the electronics of the system,

FIG. 9c showing the consequences of electronics used for anti-aliasingpost-filtering, and

FIG. 9d showing total system MTF revealing the elimination of aliasingphenomena.

DETAILED DESCRIPTION

As indicated in the foregoing discourse, during contemplated clinicalutilization, a gamma camera arrangement according to the instantinvention is used to image gamma radiation eminating from a region ofradio pharmaceutical source distribution within a patient. Looking toFIG. 1, an exaggerated schematic representation of such a clinicalenvironment is revealed generally at 10. The environment 10schematically shows the cranial region 12 of a patient to whom has beenadministered a radiolabeled pharmaceutical, which pharmaceutical willhave tended to concentrate within a region of investigative interest.Accordingly, radiation is depicted as emanating from this region 12 asthe patient is positioned upon some supporting platform 14. Over theregion 12 is positioned the head or housing 16 of a gamma camera.Housing 16 is pivotally supported at 18 from a beam 20. Beam 20,carrying a counter-weight 22, is pivotally supported at 24 in dual axisgimbal fashion from an upstanding support 26. Support 26 is fixedlyattached to and extends from a base member 28. As is represented only indotted line and generalized fashion, the head 16 is configured to retainan ultra-pure germanium orthogonal strip type semiconductor detector 30as well as resistordivider networks tapping the detector andpreamplification stages (not shown in FIG. 1) within a vacuum chamber32. Chamber 32 is retained at a predetermined low temperature, forinstance 77° K. by an appropriate cryogenic system during operation ofthe head 16 to provide one aspect of necessary detector and electronicnoise diminution. Adjacent to the detector 30 and disposed intermediatethe detector and the patient-retained source of radiation 12, is amulti-channel collimator 33, the design and structure of whichrepresents a highly important aspect of the instant invention.

During the operation of the gamma camera, radiation emanating fromsource 12 is spatially coded initially at collimator 33 by attenuatingor rejecting off-axis radiation representing false image information.That radiation passing collimator 33 impinges upon detector 30 and asignificant portion thereof is converted to discrete charges or imagesignals. Detector 30 is so configured as to distribute these signals toresistor chains as well as select preampliciation stages retained withinchamber 32 to provide initial signals representative of image spatialinformation along conventional coordinate axes as well as representingvalues for radiation energy levels. This data then is introduced, asrepresented schematically by line 34, to filtering and logic circuitrywhich operates thereupon to derive an image of optimized resolution andveracity. In the latter regard, for instance, it is desired that onlytrue image information be elicited from the organ being imaged. Ideally,such information should approach the theoretical imaging accuracy of thecamera system as derived, for instance, from the geometry of thedetector structure 30 and collimator arrangement 33 as well as thelimitations of the electronic filtering and control of the system.Instrumentation for achieving the latter function is described, forinstance, in detail and the following publication which is incorporatedherein by reference:

Xx. gerber, M. S., Miller, D. W., Gillespie, B., and Chemistruck, R. S.,"Instrumentation for a High Purity Germanium Position Sensing Gamma RayDetector," IEEE Trans. on Nucl. Sci., Vol. NS-22, No. 1, February 1975,p. 416.

Image spatial and energy level signals from line 34 initially, areintroduced into Anti-Symmetric Summation and Energy Level Derivationrepresented at block 36. As is described in more detail later herein,the summation carried out at block 36 operates upon the charges directedinto the resistive chains or networks associated with the orthogonallogic structuring of detector 30 to derive discrete signals or chargevalues corresponding with image element location. Additionally,circuitry of the function of block 36 derives a corresponding signalrepresenting the energy levels of the spatial information. The output ofblock 36 is directed to Filtering Amplification and EnergyDiscrimination functions as are represented at block 38. Controlled froma Logic Control function shown at block 40, function 38 operates uponthe signal input thereto to accommodate the system to parallel andserially defined noise components through the use of Gaussianamplification or shaping, including trapezoidal pulse shaping of datarepresenting the spatial location of image bits or signals. Similarly,the energy levels of incoming signals are evaluated, for instance,utilizing single channel analyzer components controlled by logic 40 toestablish an energy level window for data received within the system. Inthis regard, signals falling above and below predetermined energy levelsare considered false and are blocked. From Amplification andDiscrimination stage 38 and Logic Control 40, the analyzed signals aredirected into an Information Display and Readout Function, as isrepresented at block 42. Components within function block 42 willinclude display screens of various configurations, image recordingdevices, for instance, photographic apparatus of the instant developingvariety, radiation readout devices and the like, which are controlled atthe option of the system operator.

Looking to FIG. 2, an exaggerated pictorial representation of a portionof detector 30 is revealed. Detector 30 may be fabricated from p-typehigh purity germanium by depositing an n-type contact on one face and ap-type contact on the opposite face of a rectangular planar crystal.Accordingly, a high purity germanium region of the crystal, as at 42,serves as an intrinsic region between p-type sem-conductor regioncontacts 44 and n-type semiconductor region contacts as at 46. Theintrinsic region 42 of the p-i-n detector forms a region which isdepleted of electrons and holes when a reverse bias is applied to thecontacts. Grooves as at 48a-48care cut into the continuous p-typecontact or region at one face of the detector to form strips of isolatedp-type semiconductor material. On the opposite face of the detector,orthogonally disposed n-type semiconductor strips similarly are formedthrough the provision of grooves 50a- 50c. Configured having thisgeometry, the detector 30 generally is referred to as an orthogonalstrip detector or an orthogonal strip array semiconductor detector. Theelectrode strips about each of the opposed surfaces of detector 30,respectively, are connected to external charge splitting resistornetworks revealed generally at 52 and 54. Resistor network 52 is formedof serially coupled resistors 56a-56e which, respectively, are tapped attheir regions of mutual interconnection by leads identified,respectively, at 58a-58d extending, in turn, to the orthogonal strips.The opposed ends of network 52 terminate in preamplification stages 60and 62, the respective outputs of which, at 64 and 66, provide spatialoutput data for insertion within the above described summation andenergy level derivation function 36 to provide one orthogonal orcoordinate output, for instance, designated as a y-axis signal.

In similar fashion, network 54 is comprised of a string of seriallycoupled resistors 68a-68e, the mutual interconnections of which arecoupled with the electrode strips at surface 46, respectively, by leads70a-70 d. Additionally, preamplification stages as at 72 and 74 provideoutputs, respectively, at lines 76 and 78 carrying spatial data orsignals representative of image information along an X axis or axisorthogonally disposed with respect to the output of network 52.

With the assertion of an appropriate bias over detector 30, an imagingphoton absorbed therewithin engenders ionization which, in turn, createselectron-hole pairs. The charge thusly produced is collected on theorthogonally disposed electrode strips by the bias voltage and suchcharge flows to the corresponding node of the resistor networks 52 and54. Further, this charge divides in proportion to the admittance of eachpath to the virtual ground input of the appropriate terminally disposedpreamplification stage. Such charge sensitive preamplification stageintegrates the collected charge to form a voltage pulse proportional tothat charge value. Assigning charge value designations Q₁ and Q₂,respectively, for the outputs 78 and 76 of network 54 and Q₃ and Q₄,respectively, for the output line 64 and 66 of network 52, theabove-noted Summation and Energy Level Derivation functions for spatialand energy data may be designated. In this regard, energy information isderived from the sum of the signals Q₁ and Q₂ or signals Q₃ and Q₄. Thisdetermines the total charge collected on one set of strips which isproportional to the energy of the photon detector interaction.Antisymmetrical summation is utilized to generate spatial informationthrough subtractive logic. For example, the x-channel spatial signal isobtained by subtracting Q₁ from Q₂, an x-channel signal of zero voltscorresponding to an interaction which occurred below the middleelectrode strips. Similarly, the y-channel spatial signal is obtained bysubtracting Q₃ from Q₄. The spatial channels of the imaging system usedifferential Gaussian-trapezoidal pulse shaping amplifiers, while theenergy channel operating in conjunction with the Logic Control energydiscrimination function described in connection with block 40, utilizesGaussian pulse shaping and summing in carrying out requisite imagingcontrol. As noted above, the operational environment of the detector 30as well as the charge splitting resistor networks 52 and 54 andassociated amplification stages is one within the cryogenic region oftemperature for purposes of avoiding Johnson noise characteristics andthe like.

As a prelude to a more detailed consideration of the spatial resolutionof gamma radiation impinging upon the entrance components of the gammacamera, some value may be gleaned from an examination of more or lesstypical characteristics of that impinging radiation. For instance,looking to FIG. 3 a portion of a patient's body under investigation isportrayed schematically at 90. Within this region 90 is shown aradioactively tagged region of interest 92, from which region the decayof radiotracer releases photons which penetrate and emit from thepatient's body. These photons are then spatially selected by thecollimator 33 and individually detected at detector 30 for ultimateparticipation in the evolution of an image display. The exemplary pathsof seven such photons are diagrammed in the figure, as at a-g, forpurposes of illustrating functions which the camera system is calledupon to carry out. In this regard, the function of collimator 33 is toaccept those photons which are traveling nearly perpendicular to thedetector, inasmuch as such emanating rays provide true spatial imageinformation. These photons are revealed at ray traces a and b, showingdirect entry through the collimator 33 and appropriate interactioncoupled with energy exchange within detector 30. Photon path c is amisdirected one inasmuch as it does not travel perpendicularly to thedetector. Consequently, for appropriate image resolution such pathrepresents false information which should be attenuated, asschematically portrayed. Scattering phenomena within collimator 33itself or the penetration of the walls thereof allows "non-collimated"photons, i.e. ray traces d and e, to reach the detector. Photon pathtrace f represents Compton scattering in the patient's body. Suchscattering reduces the photon energy but may so redirect the pathdirection such that the acceptance geometry of the camera, includingcollimator 33, permits the photon to be accepted as image information.Inasmuch as the detector 30 and its related electronics, as discussedgenerally above, measure both the spatial location and energy of eachphoton admitted by the collimator, the imaging system still may rejectsuch false information. For example, in the event of a Comptonscattering of a photon either in the patient or collimator, the energythereof may have been reduced sufficiently to be rejected by the energydiscrimination window of the system. Photon path g represents acondition wherein detector 30 exhibits inefficient absorptioncharacteristics such that the incident photon path, while representingtrue information, does not interact with detector 30. As in apparentfrom foregoing, each of the thousands of full energy photons which areabosrbed at detector 30 ultimately are displayed at their correspondingspatial location on an imaging device such as a cathode ray tube to forman image of the source distribution within region 92 of the patient. Ofcourse, the clinical value of the gamma camera as a diagnostic implementis directly related to the quality of ultimate image resolution.

As is revealed from the foregoing discourse, the imaging resolution ofthe camera system is highly dependent upon the quality of collimationexhibited at the entrance of the camera by collimator 33. Generally,collimator 33 is of a multichannel, parallel-hole variety, itsperformance being dictated by its fundamental geometric dimensions, thematerial with which it is formed, and the technique of its fabrication.Referring to FIGS. 4a - 4c, a designation of the geometric aspects ofcollimator 33, as such aspects relate to photon path travel, and spatialintensity distribution over the corresponding spatial axis of detector30 are shown schematically. FIG. 4b shows the photon intensitydistribution at the mid-plane 30' of detector 30 due to a line source ofradiation at distance B from the collimator 33 outwardly disposed planedefining side. Note that the source position is designated "L". Sourcepoint L is located, for purposes of the instant analysis, within a plane94 lying parallel to the outwardly disposed plane defining side ofcollimator 33 as well as its inwardly disposed plane defining side andthe plane defined by the midpoint 30' of detector 30. The intensitydistribution pattern of photons, revealed in FIG. 4b, is provided underthe assumption that the collimator 33 is fixed in position. FIG. 4a, onthe other hand, assumes that the collimator 33 moves during an exposureand produces, in consequence, a triangular intensity distributionpattern of photons. A location of value "R" designates a full width athalf maximum (FWHM) spatial resolution. Such spatial or positionresolution capability of the camera system may be defined utilizingseveral approaches. However, for the latter designation, FWHM, isderived from a consideration that if a very small spot of radiationexits at the object plane, the image generally will be a blurred spotwith radially decreasing intensity. The position resolution then isdefined as twice the radial distance at which the intensity is half ofthe center intensity.

Looking in particular to FIG. 4c, considering the similar triangles EFGand LMN, the resolution of collimator 33 generally may be expressed as:

    R.sub.c = (D/A.sub.E) (A + B + C)                          (1)

where

A = the collimator thickness,

A_(E) = the effective collimator thickness due to septial penetration,

B = the source to collimator distance,

C = the collimator to detector midplane distance and

D = the effective diameter of each channel within the multi-channelcollimator

Effective diameter, D, is considered to be the square root of thecross-sectional area of a given collimator channel multiplied by 1.13.

The effective collimator thickness is given approximately by:

    A.sub.E = A - [2/μ(E)]                                  (2)

where μ (E) is the attentuation coefficient of the collimator materialat a photon energy, E.

For a given collimator material, sufficiently thick septal walls arerequired to reduce the number of photons or gamma rays that enter withina given collimator channel, penetrate the septal wall thereof and exitthrough an adjacent or other channel opening. Looking to FIG. 4c, onesuch gamma ray or photon path is traced as UV. Note, that for thiscondition, the photon or ray passes through a collimator vane or channelside of thickness, T, along a minimum septal distance, W, therebyallowing the ray or photon to exit from a channel adjacent the channelof initial entrance. The fraction of photons or rays traveling UV thatactually penetrate the septal wall is given by the penetration fraction:

    P = exp (-μ(E) W).                                      (3)

it is considered the practice of the art to design the collimatorstructure such that the penetration fraction, P, is given a value lessthan about 5%. In this regard, mention may be made of the followingpublication:

Xxi. h. o. anger, "Radioisotope Cameras," Instrumentation in NuclearMedicine, G. J. Hine, ed. Vol. 1, Academic Press, New York, 485-552(1967).

The minimum septal distance, W, is found from the similar triangles IJKand UVY approximately as: ##EQU3## by assuming A is greater than 2 D + Twhere T, as noted above, is the septal wall thickness. Solving equations(3) and (4) for the septal wall thickness, T, gives: ##EQU4## The value,T, as set forth in equation (5) serves to define that minimal septalthickness for collimator 33 which is required for a given penetrationfraction, P.

The geometric efficiency of the collimator is defined as the ratio ofthe number of gamma rays or photons which pass through the collimator tothe number of photons or gamma arrays emitted by the source. Describedin terms of the collimator parameters, such efficiency may be given by:##EQU5## where K = 0.238 for hexagonally packed circular holes and 0.282for square holes or chambers in a square array.

As described above, the clinical value of the gamma camera imagingsystem stems principally from the systems capability for achievingquality image resolution. Given the optimum image resolution which ispractically available, it then is desirable to provide a design whichachieves a highest efficiency for that resolution. For a collimatordesign, it is desirable to provide a low septal penetration fraction aswell as a practical fabrication cost. Further, an inspection ofequations (1) and (6) given above for collimator resolution andgeometric efficiency, respectively, reveals that as resolution isenhanced, the efficiency of the collimator is diminished. In accordancewith the instant invention, it has been determined that a multi-channel,parallel-hole collimator, the channels of which are configured havingsquare cross sections represents a preferred geometric design freature.In this regard, where the latter are compared with collimator channelsformed has round holes, hexagonally packed arrays or hexagonally packedbundles of tubes all of given identical dimensions, resolution remainsequivalent, but the efficiency of the preferred square cross sectionalchannel array will be a factor of 1.4 times greater than the round holedesign, while the efficiency of the hexagonally packed bundle of tubeswill be intermediate the efficiency value of the above two designs.Consequently, as noted above, on the basis of maximum efficiency at adesired resolution, the square hole cross sectional chamber design ispreferred.

Concerning the materials which may be selected for constructing thecollimator, those evidencing a high density, high atomic numbercharacteristic are appropriate for consideration. In particular, mentionmay be made of tungsten, tantalum and lead for the purpose at hand. Theprimary criterion for the material is that of providing a short meanfree path at the photon energy level of interest. For the desirableenergy level of 140 keV, the mean free path for photon attenuation is0.012 inch in tungsten, 0.015 inch in tantalum and 0.016 inch in lead.Accordingly, for a selection based upon a mean free path forattenuation, tungsten represents the optimum collimator material.Heretofore, however, pragmatic considerations of machineability orworkability have required a dismissal of the selection of tungstenand/or tantalum for collimator fabrication. For instance, formultichannel collimators having round channel cross sections, tungstenand tantalum are too difficult and, consequently, too expensive fordrilling procedures and, in general hexagonally packed arrays providingsuch cross sections are restricted to fabrication in lead. Similarly,other designs formed out of the desired materials do not lend themselvesto conventional machining and forming techniques, the cost for suchfabrication being prohibitive even for the sophisticated cameraequipment within which the collimator units are intended forutilization.

In accordance with the instant invention, a square hole collimatordesign, fabricable utilizing the optimum material tungsten, is provided.Revealed in perspective fashion in FIG. 5, the collimator is shown tocomprise an array of mutually parallel adjacently disposed channelshaving sides defining a square cross section. These channels extend todefine inwardly and outwardly disposed sides which are mutually paralleland the channels are formed axially normally to each of these sideplanes. The highly desirable square structure shown in FIG. 5 isachieved utilizing the earlier described preferred tungsten material ortantalum, such materials normally being difficult or impractical tosubject to more conventional manufacturing procedures. However,practical assembly of the collimator array 33 is achieved through theuse of a plurality of discrete rectangularly shaped sheet members, asare revealed in the partial assembly of collimator 33 shown in FIG. 6.Referring to that figure, note that member 100 is formed as a flatrectangular sheet of height, h, corresponding with desired collimatorthickness, A. Formed inwardly from one edge of member 100 are aplurality of slots spaced in regularly recurring parallel fashion andidentified generally at 102. Slots 102 are formed having a heightequivalent to h/2 and are mutually spaced to define a pitch orcenter-to-center spacing D plus T. The slots are formed having a widthof T + e, where e will be seen to be a tolerance. When the plurality ofsheet members, for instance, as shown at 100 and 104 are verticallyreversed in mutual orientation and the corresponding slots,respectively, as at 102 and 106 are mutually internested as shown, thecollimator may be built-up to desired dimensions without recourse toelaborate forming procedures. Note that the width of slots 102 and 106closely approximates the width of each of the sheet members within thearray with a controlled allowance for tolerances. In determining thevalue for the above described pitch of the regularly recurring slotswithin the sheet members, assuming resolution criteria are met, aspacing may be selected to match the center-to-center electrode stripspacing of detector 30 or a multiple thereof so that the septal walls ofthe collimator 33 can be aligned with the less active grooves formedwithin the detector. Practical fabrication techniques are available forforming the slots as exemplified at 102 and 106. In particular, chemicalmilling or chemical machining techniques are avilable for this purpose.With such techniques, a wax type mask is deposited over the sheets to bemilled, those material portions designated for removal being unmasked.The sheets then are subjected to selected etchants whereupon the slotsare formed. Following appropriate cleaning, the sheet members then areready for the relatively simple assembly build-up of a completedcollimator. Through the use of such chemical milling techniques, desiredtolerances in forming the slots are realizable. By utilizing thecollimator structure shown in combination with optimal tungsten sheetmaterial, a computable 35 to 40 percent improvement in collimatorefficiency may be gained over round hole, hexagonally packed leadcollimators of identical dimension, as well as a 50 to 80 percentimprovement is septal penetration characteristics and an average 5%improvement in geometric resolution. The collimator fabricationtechnique and structure are seen to offer several advantages over moreconventional collimators structures. As evidenced from the foregoing,such advantages include the availability to the design of the superiorshielding capabilities of tungsten; a simplicity of component design andconsequent ease of assembly and the use of optimal square hole chambergeometry for maximum geometrical efficiency. However, to achieve optimalperformance, the assembly technique necessarily introduces small gaps atthe intersections of the septal walls of a completed collimatorstructure. These gaps exist by virtue of the tolerances required for theinterlocking fit of the septal wall and the effect of gamma raystreaming through such gaps should be considered.

In earlier commentary herein, it has been noted that a septalpenetration of five percent or less of impinging gamma radiation ispreferred for collimator design. It follows, therefore, that thestreaming factor for the particular collimator structure at hand shouldbe assigned the same configurational parameter in the interest ofdesired unity of system design. Through utilization of a geometricanalysis of a worst case condition, requisite lowest tolerance requiredfor the interlocking fit of the septal walls and for a desired source toa collimator distance can be derived. Such analysis will reveal that theslot tolerance should preferably be no more than 0.001 inch and, morepreferably, should be less than that to the extent of practical millingapplication.

In the discourse given heretofore concerning the functionalinter-relationships of collimator 33 and detector 30, no commentary wasprovided concerning the effect of the discrete electrode strips of thedetector upon ultimate image resolution. It has been determined that, byvirtue of their geometric configuration, orthogonal strip detectors,without appropriate correction, will introduce "alias" frequencycomponents into the output of the system. For instance, in a purelylinear system, the output of the camera would consist of the samespatial frequency components as the input except with the possibility ofreduced contrast. Looking to FIGS. 7a c, the aliasing phenomenon isdemonstrated in connection with an exemplary and schematicrepresentation of a strip electrode detector 130. In this worst caserepresentation, no collimator is present and the electronic resolutionis less than one strip width. Looking to FIG. 7a, a source distributionis shown as may be obtained, for instance, utilizing three discretecollimated point sources spaced at equal distances of 1.5 times thestrip spacing. The reciprocal of the periodic spacing of the componentsdepicted may be represented as, v. The source distribution shown is onewith primary frequency components of v₁ = 0 and v₂ = 2v_(s) /3. Suchsource input is provided in the instant representation inasmuch as itcombines the three qualities which accentuate an aliasing phenomenon,namely, a periodic input, 100% contrast, and a high signal-to-noiseratio.

FIG. 7b, reveals a portion of a strip electrode detector 130 having theearlier described detector region grooves aligned with respect to theinput signals depicted at FIG. 7a, The one-dimensional spatial imagewhich may be derived, for instance, from a multi-channel analyzer isshown in FIG. 7c as curve 132. By comparison, the corresponding spatialimage which would be received within a system incorporating a collimatorcapable of resolving the input signals, a detector with strip spacingsatisfying the anti-aliasing criterion and an anti-aliasing electronicchannel, is revealed at 134. This image shows no aliased components.

Looking more particularly to the aliasing phenomenon represented atcurve 132, the four lowest spatial frequency components revealed are:

1. a component at v = 0, a zero frequency component which represents theaverage value of the four peaks;

2. a component at v = 2v_(s) /3, which is the frequency equal to thereciprocal of the spacing between one of the two outer peaks and theaverage position of the two inner peaks;

3. a component at v = v_(s), which is the frequency equal to thereciprocal of the spacing between each of the four peaks; and

4. a component at v = v_(s) /3, which is the frequency equal to thereciprocal of the spacing between the two outer peaks.

The first two components above are the fundamental source components,while the second two components are aliased components of thefundamental source components centered at the first harmonic of thestrip sampling frequency.

As a preclude to considering a typical representation of the spatialfrequency response of a one-dimensional gamma camera as revealed inFIGS. 8a-d the modulation transfer functions (MTF) merit comment. Asdescribed in detail in publication (III) hereinabove, the MTF is ameasure of spatial resolution that can be defined for linear systems andwhich takes into account the shape of an entire line spread function.The rationale for such description of spatial response arises from thefact that any object and its image can be described in terms of theamplitudes and phases of their respective spatial frequency components.The MTF is a measure of the efficiency with which modulation or contrastat each frequency is transferred by the imaging system from the objectto the image. This is analogous to the temporal frequency response of anelectronic amplifier or filter. Looking now to FIGS. 8a-8d MTF isplotted against spatial frequency, v, for a series of stages within agamma camera not accommodating for aliasing phenomena. In FIG. 8(a) acollimator modulation transfer function (MTF_(c)) with FWHM resolutionof 1.33 l is revealed, i.e., the curve distribution, incorporating somehigh frequency components, is representative of the signal passed to thesemiconductor detector of the camera. FIG. 8(b) reveals the outputfrequency spectrum of the detector which is seen by the spatial channelelectronics of the camera system. An aliased frequency spectrum isrevealed, the input signal frequency spectrum being present in theoutput, centered at zero frequency and additional side bands of theprimary input component are present, centered at integer multiples ofthe strip spacing or sampling frequency, v_(s) = 1/l. FIG. 8c representsthe MTF of the electronics of the system, i.e., the transfer function ofthe spatial channel electronics, while FIG. 8d shows the product of theMTF values of the curves of FIGS. 8b and 8c. Accordingly, the curve ofFIG. 8d shows the spatial frequency response of the entire system,including the introduction of spurious spatial frequency content in thesystem MTF, represented in the figure as the bump in the frequency rangeslightly below vs.

Looking by comparison now to FIGS. 9a - d the effect of insertedcorrection on the part of the collimator design and structure of theinstant invention is revealed. In accordance with the invention, thecollimator 33 design is selected to provide an MTF prefilter to limitthe spatial frequency content seen by the detector 30 to frequenciesless than v_(s) /2. Accordingly, FIG. 9a reveals that the collimator MTFis forced to a zero value at spectrum position v_(s) /2. Such designinsures that the fundamental input frequency components and the firstharmonic frequency components centered at v_(s) do not overlap and thiscondition obtains in FIG. 9b, that Figure revealing the alias frequencyspectrum which is processed by the electronic pickoff arrangement of thecamera from the detector. The spatial channel electronics complete theanti-aliasing filter system by insuring that no spatial frequenciesgreater than v_(s) /2 are passed to the imaging system of the camera.Such post-filtering of the electronics is illustrated in FIG. 9c. Theproduct of MTF conditions represented by FIGS. 9b and 9c again arerepresented in FIG. 9d which, particularly when compared with thecorresponding FIG. 8d reveals the elimination of aliasing phenomena.

Turning now to the prefiltering or corrective functions carried out bythe collimator in controlling aliasing phenomena, it may be observedfrom the foregoing that the system resolution of an orthogonal stripgermanium detector type gamma camera is determined by the collimatorresolution, the strip width spacing, and the resolution of the spatialchannel readout electronics. The collimator is assumed to have aGaussian point spread function (PSF) and FWHM spatial resolution R_(c).In accordance with the invention, the value of R_(c) should be equal toor greater than about 1.7 (l), where l is the center-to-center stripspacing in one dimension of the detector. A more detailed discussion ofthe derivation of this value is provided in publication XIX identifiedhereinabove and incorporated herein by reference.

Looking now to the specific design parameters of the collimator of theinvention, it may be recalled that collimator resolution, R_(c), hasbeen derived geometrically at equation (1) given hereinabove. By nowsubstituting the ideal valuation, 1.7 (l) now determined foranti-aliasing prefiltering on the part of the collimator, the inventivecollimator geometry or structure may be defined. Accordingly, thecollimator is defined under the following expression: ##EQU6##

The collimator further can be defined utilizinng equation (5) above forseptal wall thickness once the values of the parameter of equation (7)are determined. Further, given the value, R_(c), for collimatorresolution and the geometric parameters determined thereby as describedabove, the collimator geometric efficiency, φ_(s), as given in equation(6) above, can be applied to further maximize the performance of thecollimator. Additionally, it may be noted that by suppressingfrequencies above v_(s) /2 input signal contributions to aliasingphenomena are accommodated for.

Since certain changes may be made in the above system and apparatuswithout departing from the scope of the invention herein involved, it isintended that all matter contained in the above description or shown inthe accompanying drawings shall be interpreted as illustrative and notin a limiting sense.

We claim:
 1. In a gamma camera type device for deriving image defininginformation of the source distribution of gamma rays providing a photonenergy level, E, of interest, said device including an orthogonal striparray semi-conductor for deriving spatial and energy level informationcorresponding with said distribution, said detector array of stripshaving a frequency vs and strip spacing, l, said device furtherincluding collimator means operatively associated with said detector andhaving an inwardly disposed plane defining side spaced from the midplaneof said detector a distance, C, having an outwardly disposed planedefining side spaced from said inward side to define a thickness, A, andspaced from said source a distance, B; the improvement wherein saidcollimator means comprises:an array of adjacently disposed channels,having internal surfaces and disposed intermediate of said inward andoutward sides, said array being configured to define a septal thickness,T, intermediate said channels, an effective collimator thickness, A_(E)= A - [2/μ(E)], and μ (E) is the attenuation coefficient of the surfacedefining material of said channels, for said energy level, E, saidchannels having a channel cross sectional area of effective diameter, D;and said collimator means having a collimator resolution, R_(c), equalto or greater than about 1.7 (l) and being configured in substantialsatisfaction of the expression:

    R.sub.c = (D/A.sub.E) (A + B + C).


2. The improved gamma camera device of claim 1 wherein said internalsurface of said array of adjacently disposed channels are configured aschannel sides defining a square internal channel cross-section.
 3. Theimproved gamma camera device of claim 2 wherein said array of channelscomprises a plurality of sheet members of height, h, each configuredhaving a plurality of mutually equally spaced, parallel slots of lengthequal to or about, h/2, and of width, w; said members being mutuallyinternested along said slots to define said array of adjacently disposedchannels.
 4. The improved gamma camera device of claim 3 wherein saidsheet members have a thickness equal to said septal wall thickness, T,and said height, h, is substantially equal to said collimator meansthickness, A.
 5. The improved gamma camera device of claim 4 whereinsaid sheet members are formed of said side defining material, saidmaterial exhibiting a said attenuation coefficient μ (E) for a saidenergy level, E, of about 140 keV.
 6. The improved gamma camera deviceof claim 4 wherein said sheet members are formed of tungsten.
 7. Theimproved gamma camera device of claim 4 wherein said slots areconfigured having a width corresponding with said sheet member thicknessplus a tolerance equal to or less than 0.001 inch.
 8. The improved gammacamera device of claim 2 wherein, for said collimator resolution, R_(c),said collimator means is configured having an optimal collimatorgeometric efficiency, φ_(s), where: ##EQU7##
 9. The improved gammacamera device of claim 2 wherein: said array of channels comprises aplurality of sheet members of height, h, each configured having aplurality of mutually equally spaced, parallel slots of length equal toor about h/2, and a width, w;said members being mutually internestedalong said slots to define said array of adjacently disposed channels;and wherein, said collimator means surface-defining material istungsten.
 10. The improved gamma camera device of claim 2 wherein:saidseptal wall thickness, T, is equal to or about: ##EQU8## wherein, P, isthe penetration fraction of said surface-defining material and has avalue about equal to or less than 0.05; said array of channels comprisesa plurality of sheet members of height, h, each configured having aplurality of mutually equally spaced, parallel slots of length equal toor about, h/2, and of width, w; said members being mutually internestedalong said slots to define said array of adjacently disposed channels;and said collimator means surface-defining material is tungsten.
 11. Theimproved gamma camera device of claim 2 wherein:said septal wallthickness, T, is equal to or about: ##EQU9## wherein, P, is thepenetration fraction of said surface-defining material and has a valueabout equal to or less than 0.05; said array of channels comprises aplurality of sheet members of height, h, each configured having aplurality of mutually equally spaced, parallel slots of length equal toor about, h/2, and of width, w; said member being mutually internestedalong said slots to define said array of adjacently disposed channels;said collimator means surface-defining material is tungsten; and whereinfor said collimator resolution, R_(c), said collimator means isconfigured having an optimal collimator geometric efficiency, φs, where:##EQU10##
 12. The improved gamma camera device of claim 2 wherein:saidseptal wall thickness, T, is equal to or about: ##EQU11## wherein, P, isthe penetration fraction of said surface-defining material and has avalue about equal to or less than 0.05; said array of channels comprisesa plurality of sheet members of height, h, each configured having aplurality of mutually equally spaced, parallel slots of length equal toor about, h/2, and of width, w: said members being mutually internestedalong said slots to define said array of adjacently disposed channels;said collimator means surface-defining material is tungsten; wherein,for said collimator resolution, R_(c), said collimator means isconfigured having an optimal collimator geometric efficiency, φ, where:##EQU12## said slots being configured having a width corresponding withsaid sheet member thickness plus a tolerance equal to or less than 0.001inch.
 13. The improved gamma camera device of claim 1 wherein saidseptal wall thickness, T, is equal to or about: ##EQU13## wherein, P, isthe penetration fraction of said surface-defining material and has avalue about equal to or less than 0.05.
 14. The improved gamma cameradevice of claim 1 wherein said collimator means surface definingmaterial is tungsten.
 15. The improved gamma camera device of claim 1wherein said adjacently disposed channels are aligned in mutuallyaxially parallel fashion and normal to said planes of said inward andoutward sides.
 16. The improved gamma camera device of claim 1wherein:said septal wall thickness, T, is equal to or about: ##EQU14##wherein, P, is the penetration fraction of said surface-definingmaterial and has a value of about equal to or less than 0.05; andwherein said collimator means surface-defining material is selected froma group consisting of tungsten, tantallum and lead.
 17. The improvedgamma camera device of claim 1 wherein:said septal wall thickness, T, isequal to or about: ##EQU15## wherein, P, is the penetration fraction ofsaid surface-defining material and has a value about equal to or lessthan 0.05; and wherein, for said collimator resolution, R_(c), saidcollimator means if configured having an optimal collimator geometricefficiency, φs, where: ##EQU16##
 18. In a gamma camera of a varietyoperative to derive image defining information of the sourcedistribution of gamma rays exhibiting a select energy level, E, saidcamera incorporating an orthogonal strip array semiconductor detectorfor deriving spatial and energy level information corresponding withsaid distribution, said detector strips having a spatial frequency,v_(s), and center-to-center strip spacing, l, an improved collimatorcomprising:a plurality of sheet members of height, h, each said sheetmember being configured having select number of parallel slots extendingfrom one edge thereof along said height a distance substantiallyequivalent to the value h/2 and mutually spaced in a regularly recurringmanner to define a slot-to-slot pitch substantially equivalent to D + Tand slot width of T + e, wherein e is a predetermined tolerancedimension; any given two of said sheet members being mutuallyinternested about a select said slot of each to form a parallel multiplechannel collimator assembly each said channel of which exhibits a squarechannel cross-section of side dimension, D, and septal thickness, T. 19.The improved gamma camera of claim 18 wherein said collimator sheetmembers have a thickness, T.
 20. The improved gamma camera of claim 18wherein said sheet material is formed from the group consisting oftungsten and tantalum; and said slots are formed by chemically etching.21. The improved gamma camera of claim 18 in which said slot pitch isselected in correspondence with said detector center-to-center stripspacing, l.